Binaural (literally meaning "two-eared") sound effects were first discovered in 1881, almost immediately after the introduction of telephone systems. Primitive telephone equipment was used to listen to plays and operas at locations distant from the actual performance. The quality of sound reproduction at that time was not very good, so any trick of microphone placement or headphone arrangement that even slightly improved the quality or realism of the sound was greatly appreciated, and much research was undertaken to determine how best to do this. It was soon discovered that using two telephone microphones, each connected to a separate earphone, produced substantially higher quality sound reproduction than earphones connected to a single microphone, and that placing the two microphones several inches apart improved the effect even more. It was eventually recognized that placing the two microphones at the approximate location of a live listener's ears worked even better. Use of such binaural systems gave a very realistic spatial effect to the electronically reproduced sound that was impossible to create using a single microphone system. Thus, quite early in this century, it was recognized that binaural sound systems could produce a more realistic sense of space than could monaural systems.
However, building a commercially viable audio system that embodies the principles of binaural sound and that actually works well has proven immensely difficult to do. Thus, although the basic method of using in-the-ear microphones has been known for many decades, the method remains commercially impractical. For one thing, even if a recording made by placing small microphones inside one person's ear yields the desired spatial effects when played back on headphones to that same person, the recording does not necessarily yield the same effects when played back for other people, or when played over a loudspeaker system. Moreover, when recording with in-the-ear microphones, the slightest movement by the subject can disturb the recording process. Swallowing, breathing, stomach growls, and body movements of any kind will show up with surprising and distracting high volume in the final recording; because these sounds are conducted through the bone structure of the body and passed on via conduction to the microphones, they have an effect similar to whispering into a microphone at point blank range. Dozens of takes--or more--may be required to get a suitable recording for each track. Attempts have been made to solve these problems by using simulated human heads that are as anatomically correct as possible, but recordings made through such means have generally been less than satisfactory. Among other problems, finding materials that have the exact same sound absorption and reflection properties as human flesh and bone has turned out to be very difficult in practice.
Because binaural recording using in-the-ear microphones or simulated heads is unsatisfactory in practice, various efforts have been made to create binaural-like effects by purely electronic means. However, the factors and variables that make binaural sound rich and three dimensional have proven very difficult to elucidate and isolate, and the debate over these factors and variables continues to this day. For a general discussion of binaural recording techniques, see Sunier J., "A History of Binaural Sound," Audio Magazine, Mar. 1986; and Sunier, J., "Ears where the Mikes Are," Audio Magazine, Nov.-Dec. 1989, which are incorporated herein by this reference.
For example, common "stereo" systems focus on one particular element that helps binaural recording systems add a sense of directionality to otherwise flat monaural sounds: namely, binaural temporal disparity (also known as "binaural delay" or "interaural delay"). Binaural temporal disparity reflects the fact that sounds coming from any point in space will reach one ear sooner than the other. Although this temporal difference is only a few milliseconds in duration, the brain apparently can use this temporal information to help calculate directionality. However, to date, virtually no progress has been made at capturing, in a commercial sound system, the full range of audiospatial cues contained in true binaural recordings. One result is that stereo can only create a sense of movement or directionality on a single plain, whereas a genuine binaural system should reproduce three dimensional audiospatial effects.
It has been theorized that the dramatic audiospatial effects sometimes produced using binaural, in-the-ear recording methods are due to the fact that the human cranium, pinna, and different parts of the auditory canal serve as a set of frequency selective attenuators, and sounds coming from various directions interact with these structures in various ways. For example, for sounds that originate from directly in front of a listener, the auditory system may selectively filter (i.e., attenuate) frequencies near the 16,000 Hz region of the audio power spectrum, while for sounds coming from above the listener, frequencies of around 8,000 Hz may be substantially attenuated. Accordingly, it has been theorized that the brain figures out where a sound is coming from by paying attention to the differential pattern of attenuations: thus, if the brain hears a sound conspicuously lacking in frequencies near 16,000 Hz, it "guesses" that the sound is coming from in front of the listener. See generally, U.S. Pat. No. 4,393,270; Blauert, J., Spatial Hearing: The Psychophysics of Human Sound, MIT Press, 1983 (incorporated herein by this reference); Hebrank, J.H. and Wright, D., "Are Two Ears necessary for Localization of Sounds on the Median Plane?", J. Acoust. Soc. Am., 1974, Vol. 56, pp. 935-938; and Hartley, R. V. L. and Frys, T. C., "The Binaural Localization of Pure Tones," Phys. Rev., 1921, 2d series, Vol. 18, pp. 431-442.
A number of audio systems attempt to electronically simulate binaural audiospatial effects based on this model, and use notch filters to selectively decrease the amplitude of (i.e., attenuate) the original audio signal in a very narrow band of the audio spectrum. See, for example, U.S. Pat. No. 4,393,270. Such systems are relatively easy to implement, but generally have proven to be of very limited effectiveness. At best, the three dimensional effect produced by such devices is weak, and must be listened to very intently to be perceived. The idea of selective attenuation apparently has some merit, but trying to mimic selective attenuation by the straightforward use of notch filters is clearly not a satisfactory solution.
In sum, binaural recording and related audiospatial effects have remained largely a scientific curiosity for over a century. Even recent efforts to synthetically produce "surround sound" or other binaural types of sound effects (e.g., Hughes Sound Retrieval.RTM., Qsound.RTM., and Spatializer.RTM.) generally yield disappointing results: three dimensional audiospatial effects are typically degraded to the point where they are difficult for the average person to detect, if not lost entirely. As desirable as binaural sound effects are, a practical means to capture their essence in a manner that allows such effects to be used in ordinary movie soundtracks, record albums or other electronic audio systems has remained elusive.
Accordingly, a basic objective of the present invention is to provide means for producing realistic, easily perceived, three dimensional, audiospatial effects. Further objectives of the present invention include producing such audiospatial effects in a manner that can be conveniently integrated with movie soundtracks, recording media, live sound performances, and other commercial electronic audio applications.